This invention relates to a twin fabric hybrid forming section for use in a paper making machine. In a hybrid forming section the stock jet is ejected from a headbox slice onto a first forming fabric that is travelling in a horizontal plane in the machine direction over a series of dewatering boxes comprising a conventional open surface single fabric forming section. A second forming fabric is then brought into intimate contact with the exposed upper sheet surface at the beginning of the hybrid two fabric forming section. The partially formed sheet and the undrained stock is sandwiched between two forming fabrics; drainage then occurs through both forming fabrics. The second forming fabric is separated from the upper surface of the formed sheet at the end of the hybrid two fabric forming section and the sheet is conveyed to the press section on the first forming fabric. This invention is concerned with that portion of the hybrid two fabric forming section between the locus at which the first and second forming fabrics come together to sandwich the stock between them and the locus at which the first and second forming fabrics separate with the sheet continuing on the first forming fabric. Although the forming section described here includes a single second forming fabric section this invention is not so limited. It is common to have more than one hybrid two fabric forming section, and to have a second headbox delivering a second layer of stock onto the first forming fabric ahead of the second hybrid two fabric forming section.
In a hybrid type forming section the two forming fabrics do not follow a linear path. The fabrics together pass over a sequence of rolls and dewatering boxes which are located on alternate sides of the two fabrics and thus define the sinuous path of the two fabrics. Each dewatering box has a curved surface, which carries a group of fabric support elements, such as blades, which are in contact with the machine sides of the forming fabrics. Each dewatering box may also be connected to a source of controlled vacuum. These curved surfaces cause the moving forming fabrics to follow the desired sinuous path. The application of a controlled level of vacuum to the dewatering boxes has two effects: it promotes the removal of water from the stock between the two moving forming fabrics, and it deflects the path of the two moving forming fabrics into the gaps between the fabric support elements. This deflection of the two moving forming fabrics generates a positive pressure pulse within the stock layer sandwiched between them that creates fluid movement within the stock in the machine direction; this causes a shearing action within the stock which serves to break up fibre flocs.
The actual magnitude of each pressure pulse generated by the deflection angle of the moving forming fabrics at the edges of each fabric support element has a significant impact on the quality of the final sheet produced. The strength of the pressure pulse generated by each fabric support element should be chosen to match the stock conditions and properties at that fabric support element. Hence, there exists a need to be able to modify the strength and/or magnitude of the pressure pulses as more water is drained from the stock and the incipient paper web is formed.
Poor control of the fabric deflection within the forming section has been found to have an adverse effect on the formation process, which will in turn have a negative impact on the quality of the paper product being made.
The actual fabric deflection angle at the edge of each fabric support element in an operating twin fabric forming section has been found to be controlled by several factors. These include:
1. the geometric layout of the physical components used in the construction of the forming zone; including the element-to-element pitch for the fabric support elements, the machine direction width of the fabric support elements, and the radius of curvature of the surfaces to which the fabric support elements are attached;
2. the level of vacuum applied to the dewatering boxes which controls the degree to which the moving forming fabrics are deflected into the gaps between the fabric support elements; and
3. the amount of machine direction tension applied to each of the two moving forming fabrics.
As used herein, then following terms are to be taken to have the following meanings:
(i) the term machine direction, or MD, refers to a direction generally parallel to the direction of movement of the forming fabrics away from a headbox slice;
(ii) the term “pitch” refers to the centre to centre spacing of successive fabric support elements in the machine direction; and
(iii) the terms “fabric support element” and “fabric support elements” refer:
either to moving surfaces such as rolls over which a forming fabric moves in rolling contact,
or to static surfaces such as blades, foils or the like over which a forming fabric moves in sliding contact.
In the initial stages of sheet formation, when the level of vacuum applied to the machine side of the forming fabric, and consequently to the incipient paper web, is low, the predominant factors controlling forming fabric deflection are the geometry of the forming section and the tension applied to both of the forming fabrics. Further, although the tension applied to the two forming fabrics is usually the same, two different tension levels can be used. The two tensions are set, within the overall pattern of adjustments, to obtain the desired level of pressure pulses within the stock sandwiched between the two moving forming fabrics.
From the point at which the stock is first sandwiched between the two moving forming fabrics until the point at which the two forming fabrics separate, the consistency of the stock is continually increasing as water is drained from the incipient paper web. At the same time as the stock consistency increases, there is also a corresponding decrease in individual fiber mobility within the stock. These changes require a stronger pressure pulse to provide beneficial fiber movement which will improve the sheet properties in the incipient paper web. However, the incipient paper web eventually reaches a consistency at which no further beneficial fiber movement can occur. From that point onwards until the two moving forming fabrics separate the pressure pulse strength must be controlled by careful selection of the required vacuum level so that drainage continues, and by careful selection of the radius, fabric support element pitch and fabric support element width so that the pressure pulse strength is controlled to a level which will not act to impair formation of the incipient paper web.
During the initial sheet forming period where beneficial fiber movement can still occur, the need for a larger pressure pulse may increase at a faster rate than can be achieved by control of the vacuum level applied to the forming fabrics alone. This is because the vacuum level must be limited to a value which does not cause excessive drainage which will both reduce fiber mobility and set the sheet properties before the desired formation benefits can be achieved. It is therefore essential to obtain a larger pressure pulse by causing a higher deflection of the forming fabrics at the edges of the fabric support elements by utilizing a wider pitch between them and/or by utilizing a higher radius of curvature in the structure to which the fabric contacting fabric support elements are attached, and/or by utilizing opposed fabric support elements, such as blades, located to increase fabric deflection into the gaps between the fabric support elements.